The present invention is a method for making hydrophobic non-aggregated colloidal silicas useful, for example, as fillers in silicone compositions. The method comprises reacting an aqueous suspension of a hydrophilic non-aggregated colloidal silica having an average particle diameter greater than about 4 nm with a silicon compound at a pH less than about pH 4 in the presence of a sufficient quantity of a water-miscible organic solvent to facilitate contact of the hydrophilic non-aggregated colloidal silica with the silicon compound. The method is conducted at a temperature within a range of about 20.degree. C. to 250.degree. C. for a period of time sufficient to form a hydrophobic non-aggregated colloidal silica.
The hydrophobic non-aggregated colloidal silicas prepared by the present method are useful in many applications where fumed silicas, precipitated silicas, and silica gels are useful. However, the hydrophobic non-aggregated colloidal silicas prepared by the present method are particularly useful as reinforcing and extending fillers in organic rubbers and silicone rubber compositions including silicone adhesives, rubbers, and sealants. It is well known that cured silicone compositions, such as silicone rubbers, formed from the curing of polydiorganosiloxane fluids and gums alone generally have low elongation and tensile strength properties. One means for improving the physical properties of such silicone compositions involves the incorporation of an aggregated reinforcing silica filler, such as fumed silica, precipitated silica, or silica gel into the fluid or gum prior to curing. However, such aggregated silicas typically contain hydroxyl functionalities which cause the silicas to be hydrophilic. Such hydrophilic silicas are difficult to incorporate into silicone compositions comprising hydrophobic polydiorganosiloxane polymers and further cause the silica to react with the polymers causing a phenomenon typically referred to as "crepe hardening". A great deal of effort has been made in the past to treat the surface of aggregated reinforcing silica fillers with organosilanes or organosiloxanes to make the surface of the silica hydrophobic. This surface treatment reduces or diminishes the tendency of the silicone compositions to crepe harden and improves the physical properties of the cured silicone compositions. The present method provides a process for hydrophobing non-aggregated colloidal silica without causing aggregation of the colloidal silica.
The hydrophobic non-aggregated colloidal silicas produced by the present method are spherical in shape and can have a smaller particle size than that of aggregate silicas while providing comparable reinforcement. Therefore, the use of the hydrophobic non-aggregated colloidal silica produced by the present method can result in compositions with reduced viscosity, such as liquid silicone rubber compositions, and compositions having better clarity.
Processes for hydrophobing fumed silica by the use of, for example, organochlorosilanes, siloxanes, and disilazanes are well known in the art. For example, British Patent Specification No. 1,110,331 describes the hydrophobing of an aqueous suspension of fumed silica with an alkylhalosilane and the subsequent flocculation in a water-immiscible organic liquid.
Processes for making hydrophobic silica gels are also described in the art. Lentz, U.S. Pat. No. 3,015,645 teaches the making of hydrophobic silica powders by reacting an organosilicon compound such as dimethyldichlorosilane or trimethylmethoxysilane with a silica organogel in the presence of an acidic catalyst to form a hydrophobic silica hydrogel. The hydrophobic silica hydrogel in the aqueous phase is contacted with a water-immiscible organic solvent to convert the hydrophobic silica hydrogel to a hydrophobic silica organogel which segregates into the organic phase. More recently, Burns et al., U.S. Pat. No. 5,708,069 have described a process for making hydrophobic silica gels under neutral conditions. The method comprises two steps, where in the first step the pH of a silica hydrosol is adjusted with a base to within a range of about pH 3 to pH 7 to facilitate formation of a silica hydrogel. In the second step the silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of an acid to effect hydrophobing of the silica hydrogel. In a preferred method, in a third step the hydrophobic silica hydrogel is contacted with a water-immiscible organic solvent to convert the hydrophobic silica hydrogel into an organogel which separates from the aqueous phase.
Parmentier et al., U.S. Pat. No. 5,009,874 describe a method for making a hydrophobic precipitated silica useful as a reinforcing filler in silicone elastomers. In a first step the precipitated silica in aqueous suspension is hydrophobed with an organosilicon compound. In a second step a water-immiscible organic solvent is added to effect separation of the hydrophobic precipitated silica from the aqueous phase.
Castaing et al., Europhysics Letters 36(2) 153-8, 1996, describe the formation of a silicone elastomer, having good physical properties, incorporating a colloidal silica having adsorbed polydimethylsiloxane on the surface.